1. Field of the Invention
The present invention relates to active filter electronic circuits. More specifically, the present invention relates to active filter circuits for effectively removing noise from a power supply.
2. Description of the Related Art
Microprocessor architectures are continually evolving to improve and extend the capabilities of personal computers. Execution speed, power consumption, and circuit size are aspects of microprocessors and microprocessor performance that are constantly addressed by processor architects and designers in the ongoing quest for an improved product. As the execution speed of microprocessors has reached and exceeded 200 MHz and continues to increase, the importance of maintaining a high fidelity timing signal becomes increasingly important.
Microprocessors typically use a clock distribution system to supply timing signals to the various microprocessor operating circuits. The clock distribution system buffers timing signals and supplies timing signals having a suitably accurate frequency and duty cycle. In high speed microprocessors, the accuracy of frequency and duty cycle reproduction becomes increasingly important. One technique for supplying an accurate timing signal involves generation of a clock signal by a phase-locked loop (PLL) and distribution of the clock signal over the large area of the integrated circuit, while maintaining the clock skew to a specified tight tolerance throughout that area.
One aspect of maintaining a highly accurate timing signal is ensuring that the power supply to the PLL and the voltage-controlled oscillator (VCO) driving the PLL has a strictly limited amount of noise. Noise in the power supply signal to the VCO results in jitter that greatly reduces the effective frequency of the clock signal distributed over the large area of the integrated circuit. Present day and future microprocessor operating speeds are only attained if the noise in the power supply signal is highly limited.
Conventional integrated circuits typically use an RC filter to attenuate the power supply noise on an external dedicated VDD line coming into the integrated circuit chip. The RC filter typically includes a variable resistor or potentiometer element for manually adjusting the characteristics of the filter. One problem with the conventional integrated circuit and RC filter is that the PLL circuit typically has dual countermanding requirements of a high power requirement and a large amount of filtering to the input power supply voltage. A suitable RC filter for the demands of power supply filtering uses a large resistance and typically a large capacitance. However, a resistor suitably large for removing power supply noise is also so large that the voltage drop across the resistance effectively lowers the power supply voltage applied to the PLL excessively. Furthermore, the large capacitance of the RC filter disadvantageously increases capacitive coupling. If a large capacitor is fabricated on an integrated circuit chip, the area consumed may be prohibitively large. If the large capacitor is formed off-chip, coupling problems may arise.
What is needed is a power supply filter that removes power supply noise for high frequency operation of a circuit, such as a microprocessor, multimedia processor, or other types of circuit, so that further increases in execution speed are possible.